The extent of corrosion damage on the U.S. military's F/A-18 Hornet fleet is requiring more maintenance than expected, an admiral said.

The Navy and Marine Corps are flying the legacy fighter jets longer than planned -- 10,000 flight hours, up from 6,000 flight hours -- because of delays in the F-35 Joint Strike Fighter program, according to Navy Rear Adm. Michael Manazir, the service's director of air warfare. As a result, the 1980s-era, twin-engine aircraft is experiencing a high degree of wear and tear, including corrosion.

"The corrosion impacts, I would say, caught us by surprise," he said this week during a Navy and Marine Corps aviation conference on Capitol Hill. "When we opened them up and realized the extent of the corrosion damage, we realized we couldn't just replace the parts we were going to replace. We had to put those airplanes aside."

There are approximately 620 F-18A/D Hornets currently in service, according to a 2015 report on naval programs. The Marine Corps still flies the aircraft as a frontline fighter, while the Navy operates the plane behind the newer F/A-18E/F Super Hornets. Marines in recent months have flown F-18s to strike militants affiliated with the Islamic State in Iraq and Syria, or ISIS.

COMPOSITE VS METAL

The services had been replacing worn out parts and other components on the F/A-18 as part of a maintenance program to ensure the aircraft could reach a service life of 6,000 flight hours, Manazir said. Extending that engineering effort to yield 10,000 flight hours "would normally be a straightforward task -- if it weren't for the corrosion created by the environments we operate in," he said.

Complicating the effort was an assumption made by the Navy decades ago that the Hornet, as a composite aircraft, wouldn't require the same level of corrosion-prevention work as older, mostly metal planes, such as the F-14 Tomcat, A-6 Intruder and the A-7 Corsair II, Manazir said.

"We had not planned on operating the Hornet past 6,000 hours," he said. "So we did not do the normal corrosion control processes that we used to use on metal airplanes, like the Tomcats, A-6s, A-7s. We understood what corrosion was on metal. The science is different on corrosion on composites."

In addition, the damage to the aircraft isn't uniform and appears to be affecting different parts and structures, Manazir said. "They made each airplane coming into the depot kind of a one-off," he said. "We realized we couldn't just replace the parts -- we also had to look at the corrosion on the surrounding framework."

SITTING IDLE

Manazir didn't say how many Hornets are down for repairs at depots. But they're among an ongoing aircraft shortfall that's estimated to be more than 100 planes through 2020, he said. The problem is exacerbated by mandatory spending caps that limit available maintenance funding, he said.

"That equates to two to three squadrons," he said.

What's more, the maintenance woes are not limited to F/A-18s. The Marine Corps wasn't able to deploy CH-53E Super Stallion heavy-lift helicopters to assist with earthquake relief efforts in Nepal this spring because of a wiring issue that required replacing hydraulic lines, according to Lt. Gen. Jon Davis, deputy commandant of aviation.

Meanwhile, the Corps' F-35B jump-set variant, is scheduled to enter so-called initial operational capability, or IOC, later this year, followed by the Air Force's F-35A conventional version in the latter half of 2016, followed by the Navy's F-35C aircraft carrier variant in late 2018 or early 2019.